Telecommunication system



March l0, 1953 A. H. REEVES TELECOMMUNICATION SYSTEM 3 Sheets-Sheet 1 Filed July 21, 1948 e w t t A wie@ March l0, 1953 A. H. REEVES TELECOMMUNICATION SYSTEM Filed July 21, 1948 3 Sheets-Sheet 2 j (DEMY ,verweer Inventor Attorney March 10, 1953 A. H. REEVES 2,631,194

TELECOMMUNICATION SYSTEM Filed July 21, 1948 3 Sheets-Sheet 3 ss so s2 \59 .65 F/GZ In entor fw gym@ am Patented Mar. 10, 1953 UNITED STATES PATENT OFFICE 2,631,194 A rsrscivinomcimow srsrrny Alec Harley Reeves, London, England, assignor to International Standard Electric Corporation; New York; N. Y.; a corporation of Delaware iipiiatqiuly 21, 194s, Seria-1"- Noi sollies" l In Great Britain July 212,v 1947 solaims. (c1. 17945;)

' This; invention` relates'- tor pulse transmission systems; particularly tlios'e"I systems vvherein sig-A nals are conveyed' alonge? communication path in the form" of pulses of electrical'l energy,v the amplitude; duratiomphaseor* timing of which is varied;v that is to say, niodlLIated;y in conformity with the instantaneous" value of signals repre; sentin'gthe mfor'mationfto be'- conveyed.

In such systems@itjhas'beenarranged that the signals" from each of several substations tothe SystemA are" caused to?in o'dnlate` atV4 a central station a. pulse of electrical energy (hereinafter called a puls'e) having a? time position in a recurring cycle of pulsesi so that the pulse vtime positionsallottedv to thesbsttions recur in cyclic order;

TheV precise time atfrvhichthe pulse of a substation occursf in relationltothe cyclel of pulses may be varied With-in certain limits when the type inodulatio'nl of the pulse by' the substations signals is such as to' vary the'timi-ngof the pulse, and this modulation may vary the pulse timing about a constantly recurringmeantime position in the cycle of pulses; Furtherit has' been proposed in` United-f StatesY- application No. 756,262', led June 2 1-, 194'?,LA to" vary the timingr oi the pulse by modulation not-'inf relation toA the 'start or nish of acycleofpulses'but'inrelation to the pulseirnme'diately preceding it, allotted' to another" substation;

Where a n'rnberof substations as aforesaid are'to lbe served 'by 'a rlesser numberV of' connecting circuitsor channels, itf'isr'iecessaryf in a pulse transmission system ofthe type' described, to make any' free connecting' circuit" or' channel available to any" or the-44 substation's" rro'in which transmission is required;

According'to one of its=features""thereforev the invention consists of a pulsel time-transmission systemcomprising` means' for" selecting pulse times from alrst pulse repetition time cycle and means for transferring.modulations. carried by selectedpulse times to acorspondingnumber of pulse times ina secondfindependentl and unrelated pulse-repetitiontime cycle.v

According: toanother-of its. features,y the invention consists of a telecommunication exchange which comprises means for modulating pulses in a pulse repetition time cycle with calling or selecting signals and clearingsignals from a substation and comprising line finder means'for respending to a calling or' selecting signall toV cause selection of a Substationand means vfor respondingto a clearinglsignal onthesame 1 pulse time to-causedisconnection-fofthefsaidisubstation.

. 2 `The process of selecting a pulse time from a pulse repetition cycle isl in many ways' analogous to the functions of a line-'nder in an electromechanical switching system and the term line- 'i nder is used With reference to'. the means of effecting this selection' inrthe following description of the inv'ntionybut theterm is used solely for convenience of expression'and is not intended to' limit the" invention .toi applications such `as those carried out by linenders in the telecommunicanon field.v f Y The nature of the' invention Will now loeV de'- s'cribed relation to an" embodiment illustrated in the attached drawings in which :l

Fig. 1 is'a'loloc'k schematic' drawing of a system according to the invention;

Figs. 2, 3 and 4 are circuit-'diagrams'of alternative' 4types/'of unit representing the' rectangle' il oil' arid ITinFig'.- 1. .l

Figs. 5,' 6; ll 'nd"8- 'arev graphs of the Waveiorms at various" points of-the'circuits'illustrated in Figs. 2', 3 and 4.

In Fig'. 1,- the dotted line rectangles I; 2V and 3 are three linender unitsfconnected at points 28 andfi',l Z'Sand 30; and 3l and `32,'respectiv'ely, to aL looped'l line indicated?"byA theT reference numeral 21 Where it goes in one direction and by .the `reference nmeralv in the 'return'fdirection Anumbr of linender units'connected inf this wayto a looped line', will b'called a line-finder group.

In line'2ljffollowing the points of attachment ofthe lineflnder units I, 2 and'v 3'; are inserted delay circuits 4; 5` and'S respectively the' delay tinies` of which" are' equal to oner another but greater" thanth'e 'duration' of a pulse".

The pulsesr'epresent'ing' the various substations Connected to the systemare'introducea at ter'- minal fi, whence they'ent'er a unitl I4 to be described later,AV from which they are' applied" to linef 2-1.- Y l Delay' circzuitsI 8;. 9' and'i l0" are Vinserted in line 2llc'etvv'eerrA the points'of' connection, 28', and 2l, 29',- and 31T, and '3|' andf32"of linen'der unitsA l, 2 and'- 3 respectively; The'del'ay time` of the's'e delay circuits is less than the duration of a pulse:

Between `delayl circuits' i8; @and l0; and' connection points-'21h S'-"ani"S'ZreSpeGti/ely, unidirccticnai devices Hf,- li'ad' were? inserted in The demodulators I8, ot the y1' 4efinder are connected individually to separate scanned elements of a continuously scanning distributor device I5 which will usually be common to a number of linender groups and will have its scanned elements occupied by connections from linender units in these groups similar to that illustrated in Fig. 1. Though in certain circumstances it may serve one linender group only this will usually be undesirable for economic reasons.

Fig. 1 shows only three lineiinder units I, 2

and 3 but of course there may be more of them in a group.

Each group serves a number of substations to the system Whose signals are applied to terminal 'I and there will in general be fewer linefinder units in the group than there are substations served by the group. The ratio of linefinder units to substations will be based on the anticipated calling rate and may be of the order of 1:10 for instance.

A source of pulses provides a recurring cycle of pulses and each pulse of a cycle constitutes a channel allotted to one of the substations to the system, the communication signals of each substation being caused to modulate only the recurring pulses allotted to that substation.

There are many Ways known to the art of allocating a number of substations to individual pulse times in a recurring cycle of pulses, one method being to connect them to individual target elements of a cathode ray tube distributor of the type described in United States application, Serial No. 628,613, iiled November 14, 1945. A distributor such as that described in United States application, Serial No. 777,818, iiled October 3, 1947, may also be used.

In the embodiment described, the modulation of the pulses by the communication signals of the substations takes the form of time modulation L.,

either about a reference time which is constant in relation to the pulse cycle or with the pulse of another channel as the reference points. A'Ihe calling and clearing down conditions, are however, impressed upon the pulses in the form of duration or amplitude changes of xed magnitude.

In the absence of signals from any of the substations, the pulse trains arecontinuously applied to terminal 'I of the linender group and are of an amplitude level which will be hereinafter called level A.

On the commencement of a call by a substation it is arranged that one pulse on that substations channelshall have a greater amplitude than level A which will be hereinafter called level B. After this one pulse, the pulses of this channel then continue on level A.

It may be arranged that pulses of dierent duration are substituted for pulses of difference level, for this calling process, and it may further be arranged either that the linender units respond to a pulse of increased duration or alternatively that such increased duration is converted into an increased amplitude between terminal 'I and the connections to the linender units.

Since there are many ways known to the art, of varying the amplitude duration, or timing of a pulse allotted to a substation in a recurrent pulse cycle, in the form of modulation, the provision of these difference types of pulse for calling etc. which is also of the nature of modulation, present no diculty and as no claim is made to the means of modulating the pulses, they will not be described.

The operation of the group will rst be de- 4 scribed on the assumption that all linender are free and the description will relate to lineinder unit I as representative of any linender unit.

The combined functions of gate circuit I6 and selector Il are a. To respond initially to a pulse on level B from a calling channel,

b. To continue to respond thereafter to pulses on the same channel of level A but to be unresponsive to pulses of other channels of any level,

c. To pass a pulse on the same channel, once per cycle, to thedemodulator I8, varying in timing to the extent of any time modulation of the pulse by the substations signals, and

d. To remain responsive to the pulses of the said channel provided that there is no break in the regular arrival of one pulse of level A or greater in every pulse cycle.

The functions of gate 'circuit I6 and selector II may be combined in one device which may take the form of a time measuring circuit, capable of being started from the quiescent state by a pulse of level B only, and thereafter requiring to be re-triggered once per pulse cycle by a pulse of level A or greater. It must be unresponsive to intermediate pulses on other channels.

With such a device, a pulse can be generated in the device or a gate may be opened permitting the passage of the channel pulse from line portion 2? at the appropriate timing, such pulse being passed to demodulator I8.

Alternatively the one device may consist of a counting circuit which initially starts to count to a radix equal to the number of pulses in the pulse cycle, on receipt of a pulse at level B only and thereafter continues to count pulses of any level above a chosen minimum but ceases to count when the pulse of the channel to which it is synchronised falls below level A, to a lower level C, to be described later. A recurring pulse at the timing of the pulse of the channel in question will be passed to demodulator I8.A

t will sometimes be desirable however, to have separate circuits for I6 and I' and in such cases I may be a gate circuit initially triggered by a pulse at level B and when operating, it may either pass a pulse to I'I once per pulse cycle on the channel of that B pulse, so long as it receives pulses regularly on that channel once per cycle, of level A or greater. Alternatively it may pass the pulses of all channels so long as it is periodically triggered by a pulse of leve1 A or greater, cn the channel to which it is synchronised.

In such cases, I 1 may be a time measuring circuit or a counting circuit capable of passing a pulse to demodulator I8, at the timing of the required channel, once per cycle so long as I 6 continues to operate.

Circuits for the purpose will be described hereafter by Way of example. The circuits described in United States application Serial No. 777,761, filed October 3, 1947, as a delay circuit and a delay gate are used ina comparable role and could be used also in the role of gate circuit I6.

Demodulator IS is 'a pulse time demodulator and may be of any known type, for instance, when the pulses are modulated in relation to a constant, time reference point, it may include a double stability pulse gate in which case it may be arranged that a series of evenly spaced pulses hereinafter called master pulses of the same frequency as the channel pulses, are applied to it having a timing so that they arrive at demodulator I8 slightly after the latest time of arrival essere@ "ofY achannel pulse 'delayedto the maximum extent by modulation. It could then be arranged for the selected channel pulse to operatethe gate in one sensev and the lsucceeding master pulse to operate it in the other sense, the distance be tween the two being converted by integration equipment also forming part of It, into signals corresponding to the signals of the calling channel, without the pulse component, that, is, in the case of speech signals for instance, to continuous 'audio frequencies.

These master pulses could be applied to' de-A modulators I8 of the linender units via a terminal IS and connection 33.

Demodulator I8 has a connection to one scanned element of distributor I and when the process so far described has been completed, the calling substation has been allotted to one of the several possible outlets from distributor I5 as will now be described. Distributor I5 may be an electronic distributor of the cathode ray tube type as described in United Statesapplication Serial No. 628,613, led November 14, 1945, cr of the multi-gap cold cathode tube type as described in United States application Serial No. 777,818, led October 1947.

The output from distributor I5, emerging at terminal 23, consists of pulses, recurring in a repetitive cycle, each cycle having a number of pulses depending upon the number of pulses of scanned elements in the distributor. Each pulse of a cycle is individual to a linefinder unit and when a calling substation has been connected to a unit, in the manner above described, the corresponding pulses in each cycle of distributor I5 'will' be modulated by the signals applied to the corresponding scanned` element of distributor I5 from demodulator I8 of'the linender unit.

It will be observed that the pulse emerging from the common element of distributor I5 has been transferred to a different timing in a different cycle of pulses which need bear no relation to the original cycle of pulses entering at terminal 1. As there are fewer linender units than there are substations it follows that a time cycle of a given number of pulses can be used, beyond the linefinders, to serve a much larger number of substations If the ratio of 10 to l, substations "to-linefinder units, is adopted the number of Vsub'- stations which can be served beyond the line- 'ii-nder units for a given speed of pulse repetition,

has been multiplied by 1G, as compared with the subs-tations which could be served by the same speed of pulse repetition if each substation had an individual time position permanently allotted to it, in the pulse cycle at terminal 23.

It is necessary to ensure that the 'calling pulse at level B operates only the one linender unit.

Pulse generator 2o is arranged so that af-pulse of level B applied to it via IS and I'I causes it to generate a pulse of an amplitude equal to the difference in amplitude between level A and level B; but of opposite polarity. This pulse is applied tothe looped line 2.211V at point2-I.V It is arranged that the delay period of delay circuit 8 is equal to the-overall 'delay imposed upon pulses in their `passage through I5, I? and 2Q in the linefinder unit, so that the pulse from' pulse Vgenerator 20 coincides exactly in time with the B level pulse vvto which it owes its origin, when the said B pulse 'has travelled by the direct'path along looped line 21, through delay circuit 8 and unidirectional 'device II. Beyondpoint 2| therefore, there will heno Blevelipulsea's .it willihave been reducedlto :levelvA vby .the opposing pulse lfrom 12'0; There- 75 pulse cycles applied to terminal 'I'.- It will, therefore, be clocked by rgate-i circuit It or linefindciunit I and will therefore not be neutralised by an opposing pulse from pulse generator 20 of that line'nder unit. It will, therefore, pass, via 8, H, and 4 to connection point 29 and gate circuit It of linelnder unit 2, whichl will respond to and synchronise with it.

The same sequence of operations as that above described in relation to the irstcall will now take place in lir-iendei unit 2, and connection to element 23 of distributor i5 will be established.

If a third call originates while l'i-nender units I and 2 are still occupied, it will'be found by linender 3, and so on down the line of linender units in the group, from Ileft toY right in Fig. l.

If, however, lineinder unit I becomes free and a fourth call originates, whilst line -nder units 2 and 3 are still occupied, lineiinder unit i will nd this fourth call as its gate circuit It is free to respond to the B- level pulse.

It is thus seen that an incoming call is found" by tle rst free linender unit `from the left, in Fig.

When a calling substation terminates the call, it is arranged that one or morel pulses on its channel are reduced from level to a lower level C. This level isv insufficient to 're-trigger gate circuit i6 of the-linender unit it has been using and this gate circuit reverts to its normal unresponsive state frorn which it can only be changed by a new calling vpulse at level B.

The description now reverts to the rst call labove described. The channel pulses at level A, which succeed the original calling pulses at level B, reach: the Vpoint 2I after passing through de lay circuit 8 and unidirectional device II and will meet no opposing pulses'from pulse genera'- tor 20, at connection point 2 I. Thence they will pass from left to right along the outward going portion 21 of the looped line, pass-ing through the delay circuits 4, 9, 5, I0 and 6 and unidirectional devices I2 and I3 in the order 4, 9, I2, 5, I0, I3 and 6.. They will then return to a unit IIE to be described later along thehomecoming por- 4tion 25 of the loopedline' andivia -a pulse repeater circuit 26, also to be described later.

The returning pulses have been delayed by the delay circuits 8, 4, 9, 5, |10 and 6, and by the looped line itself intheirv passage round the loopedline and it is arranged-that each shall coincide, at the point 2,8, with a later pulse of the same channel.

The unit I4 ensures'this. I"he said delay circuits-and looped linearearrang'ed to have a total 'delay equal as nearly' as possible to the pulse cycle time, lor a multiple-thereof, so that if possible some part of a returning pulse will coincide with-'some pfartf'o'f a later pulse of the same channel. This is capable of achievement when the number of channels is small, withoutv unduly close tolerances for 4the-various circuits through fwhichfthe-pulsesfpass. 1

Where this is the case, unit I4 can consist of a pulse gate, in the form, for instance, of a limiting amplier to which the direct pulse input from terminal and the returning pulse from portion 25 of the looped line are applied in series aiding. Such an amplifier would have a standing bias such that where both pulses were at level A a single pulse at level A would be produced in the output, but where one of the pulses is at level B and the other at level A an output at level B would be produced. An original calling pulse from terminal l, will, therefore seize a linender unit if one is disengaged, and will return along the homecoming portion 25, of the looped line as an A level pulse, due to the neutralising action of 2o in the linender unit concerned. The original B-level pulse will meet a previously transmitted A-level pulse in unit I4 and a B-level pulse will be sent to the lineiinder units. A pulse at level B can only be received over the homecoming portion 25 of the looped line if a call is received when all the linender units of the group are occupied and there is little likelihood of it coinciding in unit I4 with a B level pulse received via terminal 1 since the indirect B level pulse will coincide with a later direct pulse from terminal 'l which will have reverted to level A after the single calling pulse at level B, in which case the circuit would still send on a B pulse in response to the simultaneous B pulses received. The operation necessary to produce the calling signal at the substation, would have to be repeated twice in succession at pulse channel frequency to cause consecutive B pulses and if it is a manual operation the likelihood is so remote as to be ignored but with an automatic call device, precautions to obviate the possibility may be desirable. When all the linender units are thus engaged, the returning AB-level pulse from 25 will cause a B-level pulse to issue from unit I4 and this B-level pulse will continue to circulate round the looped line till a linefinder unit becomes free and finds it. After this, it will be neutralised by 20 of the linefinder unit concerned and will return to unit I4 over line portion 25 at level A.

If the unit I4 receives a pulse or pulses at level C from terminal 1 they will be combined with returning pulses at level A, from line portion 25 as it is arranged that C level pulses are reconverted to A level pulses by amplifier 26 for reasons and in manner to be explained.

The combination of a level C pulse and a level A pulse in unit I4 results in an output at level C which fails to trigger gate circuit I6 of the linender unit hitherto occupied. Several C pulses may be transmitted to clear down the connection to make sure that the linender unit does not accidentally retrigger. It is necessary to ensure however that the C level pulse does not return at level C along line portion 25 because level C pulses would then issue from unit I4 even after level A pulses were resumed at this channel timing from terminal "I, and this would continue indefinitely so as to debar the subscriber allotted to the channel in question, from making another call.

The amplifier 26 is arranged to cater for this requirement. It may be a pulse amplifier having two limiting peak output amplitudes; at level A for a received pulse at or between levels A and C and at level, B for an input above level A.

It is necessary to insert amplifier 26 to full the function just described and also to stabilise the level of pulse returning along line portion 25 and restore any loss of amplitude incurred en route round the looped line and the iiarious 111-' serted delay circuits etc.

It may be necessary to alter the timing and duration of the returning pulses from line portion 25 so that coincidence with some part of a succeeding pulse on the saine channel is achieved. Time modulation of the pulses might otherwise alter their position suiciently to cause a failure of coincidence in unit I4 without this precaution, This may be done in ampliier 26 or unit I4 by well known means and it is preferable to provide a square pulse of a duration such that it just cannot overlap the pulse (from terminal l) of another channel with maximum modulation time displacement of the latter and having regard to variations of timing of the square pulse due to manufacturing tolerances.

It is necessary to prevent any feeding back of pulses in the reverse direction (anticlockwise round the looped line as drawn in Fig. l) and accordingly the unidirectional devices II, I2 and I3 are inserted. They may be metal rectiers for instance.

Pulses of reduced width, may, if desired, be used instead of pulses of the reduced amplitude C and in this event the lineinder units would have to differentiate between pulses of different duration or else such different pulse duration could be converted to different amplitudes between entry at terminal I 'and the connection points of the linender units to portion 21 of the looped line. This could be done by unit I4.

So far the embodiment of the invention has been described in its broad outline. Its practical details, however, will depend upon the conditions in which it is to be used.

Three main sets of conditions are envisaged and several forms of apparatus suitable under these conditions, for carrying out the functions of the various elements shown in Fig. 1, will be described by way of example.

The three sets of conditions are as follows:

1J Where there is a relatively small number of substations incoming to a linefmder group each having a pulse timing varying, when modulated by communication signals, about a constant mean time position in relation to the pulse cycle serving these substations. These circumstances will be referred to as case l.

2. Where there is a large number of channels with modulation of the pulses about a constant mean time position as in l." These circumstances will be referred to as case 2.

3. Where the timing of the pulses may be varied, by modulation, in reference to a datum which is not constant in relation to the cycle of pulses (for instance as described in United States application, Serial No. 756,262). These circumstances Will be referred to as case 3.

In case l, the lineinder units may have the function of delay circuit I6 and/or selector I1 fulfilled by a simple triggered delay device having a restoring time slightly less than the recurrence periods of the pulses of any one channel but not so short as to be capable of being triggered by the pulse of a preceding channel.

In commercial practice a 1% tolerance would be adequate to meet this requirement in the case of a linelinder group serving about 10 substations and this should not be diicult of attainment.

In case 2, which might be taken to apply to linender groups having more than about 10 incoming channels.- there would be considerable facevi-ics 9;- difculty measuring the necessary reliability of operation if -a trigger delay device were to be used and a counter circuit would be preferable in the role of gate circuit I or selector I1 (or both), since it would record the pulses by counting and would be independent of their timing.

In case 3, a counter circuit is essential for IE and/or il, as the timingl of pulses of different channels would be certain to overlap between successive cycles unless the time spacing between pulses was made extremely large in relation to the pulse width.

Circuits suitable for case l will now be described.

It has already been indicated that a delay circuit such as that described in United States application Serial No. '777,7 61 could be used as delay circuit IS and/or selector il in a linender unit. Other devices using cold cathode gaseous discharge tubes are illustrated in Figs. 2, 3 and 4.

In these three figures corresponding items have been given corresponding reference numerals.

Figs. 5, 6, 7 and 8 are graphs of the wave orms at various points in the circuits illustratedrin Figs. 2, 3 and 4. Corresponding things having the same numerical references throughout these four Figures 5, 6, '7 and 8.

A simple circuit, using a gas tube is shown in Fig. 2. The cold-cathode gas-filled diode 33 has an anode 34 and a cathode 35;. 34 is connected via resistor 35 to the positive end of a battery 31, the negative end of which is grounded. Cathode 35 is connected via resistor 38 to ground. Resistor 38 is shunted by condenser 39. The battery 3T is adjusted so that the gas tube will not lire without the assistance of the positive pulse next referred to.

At time 5i! of Fig. 5 a positive pulse arrives from the incoming line at terminal 40 of Fig. 2 and is applied to the anode 34, via condenser 4I. The gas tube iires and the voltage at cathode 35 will rapidly rise from zero to a level 53 as shown in Fig. (i. The rise in voltage lowers the effective potential across the electrodes of the tube and the discharge is immediately extinguished. This phenomenon is described in detail in our copending United States application Serial No. 19,083 48, filed April 5, 1948 wherein the voltage surge followed by the extinguishment of the tube is called a sdueg and this expression will be used hereinafter to describe the phenomenon. On extinguishment or the discharge the voltage at cathode 35 ialls exponentially to a level 65 at time 5l in Fig. 6. It is arranged for the voltage at cathode 35 to be still falling appreciably at time 5l. but at this time to be'lbelow the critical value 54 to allow the tube to be re-red by the second input pulse at or after time 5I.- The tube will thus be rre-fired, the volts at cathode 35 (Fig. 2) again rising to a level 53, when the tube will squeg once more.

The tube can be red by an input pulse anywhere between times 55 and 5l. (Figs. 6 and 5) Ifor between 51 and 52-during which intervals the cathode volts-on condenser 39 have leaked away sufficiently. The stability of the restoring time- Vconstant. will depend directly on the slope of curves 552-55, Fig. 6, at time 5 5 where retriggering is to be again possible.

In G, this slope is relatively small; thereriore it would be diiicult to meet the necessary tolerance even lor a small number ,of channels. Even a small Lchange in battery volts etc., rnight cause the tolerance gure to be .exceeded A change in the value of resistor 3B or condenser Si in the circuit ci Fie. 2, mcrecver. will sive a rst order change ill. lille vtime constantand high stability resistors and' condensers arel in some cases too expensive to be commercially practicable. The circuit of Fig. 2 is described principally because it is the basic circuit, of which those shown in Figs. 3 and 4 are improvements. It may, ofcourse, be adequate as it stands for certain applications.

A trigger circuit meeting the conditions more readily is shown in Fig. 3. The only change from Fig. 2 is the addi-tion of the anti-resonant circuit consisting of inductances 42 and condenser 43, between the cathode 35 and the ungrounded end of resistor 38. When the rst input pulse now arrives at time Eli on terminal 4l), the cathode volts will iirst rise rapidly from Zero as before; but in falling again after the squeg (due again to 3e and 39), the voltage Wave on 35 will have two components:

a. The exponential fall 53-55 of Fig. 7 (shown by a dotted line), due to leakage of condenser 39 (Fig. 3) through resistor 38; from level 5.3 to level 65, this latter level being now arranged to be of higher value than in Fig. 5 and b. The lightly damped sinusoidal wave due to shock excitation of tuned circuit 42-43 by the voltage rise. The resultant Voltage Wave at 35 is shown by the full line 58-59-5ll-5l-62-f63 in Fig. 7.

The natural half period of antieresonant circuit ft2- 43 is adjusted to be a suitable odd integral fraction of the time interval .5D to 5l (Fig. 5) approximately.

The resistor 35 has a relatively low value in the circuitof Fig. 3. On re-triggering at time 5I, therefore, the remaining energy in the damped train will be substantially damped out by the low tube impedance on ring and the circuit 42-.43 will be shock-excited again by the voltage surge when the tube res. The Wave 58-59-65-61- 52 in Fig. '7 will then be repeated.

The slope at time 64, at which this latter curve rst crosses vcritical level ,54 for the refiring of the tube by the input pulses, is now much steeper than at time 55 of Fig. 6; the time constant stability against any undesired changes in circuit constants, including changes in values ofv 38-39 will be correspondingly increased, the only factor now giving a first order change in this time constant is the natural frequency of circuit i2-33, and it is, vin general, cheaper to obtain an inductance stable within certain limits than a resistor stable within these same limits. Changes in condenser (i3 will again give rst order changes; but a high stability condenser in general is again cheaper than a high stability re- Sister. at any rate when the resistance value iS of the order of several hundred thousand ohms at least, as it mightl have to be in many applications of this device.

The level of the sine-wave ripples on curve 58-55-85-6l-52-53 may be adjusted suitably with respect to the exponential component 53-55 by suitably arranging the anti-resonant impedance of l2-4t" with respect to the impedance of the combination BS-. If the ripples .are ci 4two 19W a velue, they will have little eiectyand the result will be much the same as in curve 53-.55 in Fig, `6.v AIt they are of too high a value, or if the natural frequency of 52..-53 is too high, it may give a trough on the curve that is too near the critical level 55 :matins the input bulseiust' betere the desired one liable to trigger the tube. promise is therefore chosen.

The circuit of Fig. 3 might be expected to cater for channels in number up to about 8, with normal commercial tolerances in components.

In extension of the same expedient, it is possible to use several tuned circuits in cascade instead of a single tuned circuit.

For example, one tuned circuit may have 5 half periods in the interval Ell-5l of Fig. 5., and a second tuned circuit 15 half periods. This gives a higher slope for curve 58-59-60-6I- 62-63, without bringing this curve too near the critical level 65, on the input pulse immediately preceding the desired pulse.

A device similar to that of Fig. 3, is described in U. S. Patent No. 2,462,078.

A further improvement in time constant stability may be obtained by using the circuit of Fig. 4.

The circuit is similar to that of Fig. 2 except for the addition of certain components in parallel with condenser 39 and resistor 33.

A delay network 44 is now connected between cathode 35 and ground, via small condenser 45. The end of condenser 45, furthest from the connection to cathode 35 is connected to ground via resistor 46. The output terminals 41 of the delay network 44 are short circuited. A rectier 43 is connected in series between condenser 45 and one input terminal of the delay network 44 and a resistor 49 is connected in parallel with the input terminals of the delay network.

The rst input pulse (hereinafter called the starting pulse) at time 50 Fig. 5 will cause the tube to re and squeg as before. During the sharp rise of cathode current a sharp positive pulse will appear across i6-this will correspond to the derived curve against time of the current rise in the cathode load 38, the values of 45 and 46 being so adjusted. The starting or B level pulse at time 5D is arranged to be of higher peak voltage than the subsequent input pulses, which are of constant lower amplitude A. These subsequent pulses are arranged to be of insufficient amplitude, added to the voltage of battery 3l, to trigger the tube, even when the charge on 39 due to the sudden rise of voltage, has died away after the squeg. The time constant of 38-39 is adjusted so that this charge on 39 has died away substantially to zero in a time small compared with that between input pulses. Owing to rectifier 48 the fall in cathode current will give no negative pulse across 49, as the rectifier 43 blocks currents in that direction.

The pass band of delay network 44 (a lowpass lter of conventional design for example) and its phase characteristics, are such that after refiection at the short circuited terminals 41, the sharp positive pulse due to the sharp rise of cathode current at time 5D is transformed into a longer negative pulse of smaller peak amplitude across the input end at resistor 49, due to the low-pass characteristics of the delay network 44, and this longer negative pulse will be developed across the resistance 49 after relection from the short circuiting connection 4T, and repassage through delay network 44 in the reverse direction. The pulse will undergo a delay equal to twice that of 44. This delay is arranged to be such that the beginning of the reflected negative lpulse on 49 occurs at time 63, Fig. 8-at a time corresponding to earliest ltiming of the leading edge of the next pulse to A suitable comwhich the circuit is required to respond, for example, the next pulse of a given channel in the pulse cycles delivered to terminal l, Fig. l (advanced to the maximum extent by time modulation for instance) i. e. at time 66 in Fig. 8. The end of the reflected pulse is arranged to occur at time 61, the time corresponding to the latest timing of the trailing edge of the next pulse on the same channel (retarded to the maximum extent by time modulation, for instance).

The adjustments are such that only between times 63 and 61, when the reflected pulse arrives back at cathode 35 via rectiiier 48 and condenser 45, is the cathode suiiciently negative to enable a pulse to trigger the gas tube and intermediate pulses will have no effect. Condenser 39 has a higher value than 45 to enable substantially the whole of the reflected pulse to pass from right to left across condenser 45 without differentiation, while as the positive pulse passes across 45 from left to right only the derived pulse wave form is obtained across 45.

This circuit of Fig. 4 has two advantages over the circuits illustrated in Figs. 2 and 3:

1. The time constant of the delay network 44 is substantially the only determining factor inA the tube restoring time, and

2. This restoring time is determined only by the tube triggering time, and not as in the previous cases by the shape of the curve representing the current through the tube.

rIYnis is an important point, as the triggering time of a normal gas tube, dependent as it is on the ionisation time-constant of the gas, is always more definitely determined than the moment of extinguishing during a squeeze, which depends on the de-ionising time-constant which is normally less well determined.

The restoring stability of the circuit of Fig. 4 may thus be substantially higher than the two previous circuits. It might be used in a lineiinder group having about 20 channels, for example, without unduly expensive components, or special selection of gas tubes. The only item that has to he designed to fairly close tolerances is the network 44 of which the delay time is the most important factor.

In case 1 the selector i1 may consist, for example, of the arrangement of Fig. 3. The input (positive) pulses (every pulse in the cycle). are applied at terminal 4B, and an output pulse is taken from the cathode (also positive pulses) once per cycle. The function of gate circuit it can also be fulfilled, with no additions. Once the gas tube is extinguished it can be arranged thatI only a pulse at level B can first re it. At a channel recurrence frequency of 10 kc., which would be suitable in the case of speech signals for instance, it is found that such a tube can easily be designed so that when once fired the residual ionisation present when the second pulse of a given channel arrives, causes the tube to be fired also by pulses at the lower level A, but not at the still lower level C. If one pulse is missed, however, and there is a gap of two cycle periods before a pulse at level A or B appears (as at disengaging) the residual ionisation will have died away suiciently to prevent re-ring except by a level B pulse.

Alternatively, the arrangement of Fig. 4 may be used, in this case it is unnecessary to rely on residual ionisation for re-iring by a pulse at level A, after rst firing by a pulse at level B; the reiiected pulse from delay network 44 may easily be arranged, so that in conjunction with an input pulse at level A it will rere the tube.

The de-ionsation time may, therefore, belowered. There Will -be no reflected `pulse once a pulse has Abeen missed, as .the tube will not fire and there will be no voltage developed across 38, to `be reflected. Therefore, a level B pulse (being the starting pulse of Fig. 4), will be `needed for re-ring.

When the tube of .3 or 4 .first res (from a level B pulse), ,the output Vpulse onthe cathode will be at a higher level than at subsequent times (from level A pulses), as vthe anode has to be raised to a higher `potential 4before firing takes place.

A Vrectifier in Series with a suitable opposing battery may be shunted across 38 Yto limit the peak Avolts of the cathode pulse always to ythe same level as is Agiven by a subsequent firing from level A-othervvise in the case of Fig. 3 it Will -be dicult to keep the wave-form of the voltage across the cathode (see Fig. 7), the same in both cases, and to prevent the restoring time-constant from being changed unduly and `in the case of Fig. 4, the reflected pulse from 41 will be at a higher level on iirst firing, thus making the tube lliable to re-fire the neX-t vtime after the initial firing), before the channel signal pulse arrives or even on v a previous pulse belonging to another channel.

This added lrectifier' across resistor 38 may also be used to fulill the function of the unit r2li `of Fig. 1. Current will only iovv through it on irst firing, from a level B pulse. By means of a pulse transformer, for example, it may then be arranged that when this current pulse (which will have a constant level) occurs in the rectifier,

the required opposing voltage is applied at points 2|, 30 or 3 2 (according to the llinender unit which nds fa call), to reduce the .direct signal pulse at level B, from I I, I2 or I3 in Fig. 1 to the required level A,

It is possible also to use a low pass filter as a deinodulator I8, ,and when this is done the time modulation of the pulses from selector Il must iirst b e changed to amplitude modulation. This may be accomplished by applying .the pulses in series with a source o f sawtooth wave form, the `savv teeth `having a recurrence frequency equal to the channel .frequency and .timed so ,that the pulses occur during the sloping portion of the sawtooth wave form.. The sawtooth wave form may be produced 'by .a source common .to several linender groups and may `be applied t terminal I9 (Figli).

Cases 2 and '3 will now be considered together as, though they relate to diierent circumstances, they are capable of the Asame solution.

In case Where a large number o f channels is served by one 'linender group, it may be com.- merciclly bupracticabie to use the circuits oir-ies. 2, 3 and 4, as the timing of the moment when the tube is capable of being re-red `can only be accurately determined b y using componen-ts made to close tolerances.. If this timing were insufficiently accurate, the tube -might Well fire on the pulse of another channel and the call Would be lost.

In case 3, where the channel pulses are modulated not in relation to a constant time instant in the pulse train but in relation .to the preceding pulse belonging to another channel, it may 'be fundamentally impossible to ensure that the tube re-fires on the correct pulse. in systems using this type of modulation the pulse of a channe may vary in relation to the puise cycle, from one limit determined by the cumulative `total of the advancement of all the channel lpulses inthe pulse '14 channel, advanced to :the Yrnfexirlrium by' fr ncdulal tion, :to .another limit .determined by :the .cumulative total of .the delay of all the lchannel pulses in .the pulse cycle, delayed to Athe maximum by modulation.

The time excursion .of later impulses in the .cycle is therefore considerable, and may well occur at timings occupied at other times by the pulses of channels removed several places from the :pulse in question. In -cases 2 and 3 therefore it is desirable to 'use a pulse :counting device :the roles of gate .circuit I6 and/or .selector -H so that the pulses o f ,a given channel are identified independently of their timing `in relation to the pulse cycle The functions of I 6 and |31 can be tullilled by multi-gap cold cathode gas tube counting kdevices such as those described in my United States applications Serial Nos. 777,815 and 777,816, both nled October 3, 1947. These tubes have a vnumber of discharge gaps across which successive pulses are applied in common, one `gap being a1- r ranged .to nre vfrom the first pulse of a cycle by various alternative means described in the said speciications. Each successive pulse causes a new gap to fire due `to the spread of ionisation from -an adjoining gap fired previously. Finally -when all `gaps have fired, all the discharges are extinguished and .the process is repeated. For use Vin this invention, the tube should have a number .of gaps equal :to the :number of channels served by the linennderg-rcun .It can be arranged that the tube will `lonly lire in its'starting gap in the presence of a pulse at level B, when Once rit has I.been .extinguished lfor a time long enough for the gas in the tube to become de-ionised, and that the tube will .continue to operatein successive cycles from pulses of level A once fired initially by a pulse at level B. It can also be arranged that the tube Will not fire 0n any of its gaps in the presence of a pulse at level C,

The tube would be used either alone fulfilling the combined functions of i6 and Il, in which case it would be arranged that, once started counting by Aan initial pulse at B level, it would continue to count pulses of levels A, B or C except that a pulse at 'level C would fail to iire the starting gap and would cause the tube to stop counting,

It could be arranged for all Output pulse to 'be delivered to demodulator I8 once in every pulse cycle, so long .as the tube was counting, on the firing of the starting gap. The pulses of other halnels Would thus be counted 'but not passed "With such devices, it would be necessary to a 1- range for the working conditions to be such that the state of 'residual ionisation needed to ensure the tiring of the starting gap from an A-level pulse, when once the tube has been initially started Afrom the quiescent state by a B level pulse, was located at a fairly steep part of the curve of ionisation plotted against time so that if an A pulse arrived substantially later than the latest normal pulse recurrence -tiine the ionisation would have `fallen below the level necessary to nre the starting gap from a pulse at leveln. lf this were not done, a clearing pulse at level C would cause the startinggap to vmiss that pulse 'but it might Vwell fire from the next succeeding level A pulse of an adjoining channel, andthe linefinder unit would lock itself to that later channel and continue to operate, instead of clearing the connection.

Alternatively the devices described in my co- 15 pending United States application Serial No. 39,936/48 led July 21, 1948 may be used. These devices will now be briefly described.

Three devices are described in my said United States application Serial No. 39,936 /48 consisting of multipoint cold cathode gas discharge tubes with accompanying circuits which count pulses by means of the successive firing of a number of discharge gaps but in which no battery is required to maintain a constant potential across the gaps in the absence of pulses. i

One of these devices causes a discharge to take place on one additional gap on every successive pulse, all gaps having once iired being extinguished between pulses and re-tring on succeeding pulses due to the residual ionisation of the gas due to the recent discharge from the previous pulse. A rst pulse will re a first gap which is shorter than the others. The other gaps are dimensioned and the constants are chosen so that they will only nre from a pulse when the gas within them is ionised.

A succession of discharges takes place and every pulse fires one more gap than the last, until all gaps are red between one multipoint cathode of the tube and an anode. When these have fired and extinguished a further separate cathode hereinafter called the penultimate cathode fires from the next pulse together with all point of the multipoint cathode. The penultimate cathode will not fire off the next pulse however owing to a time constant circuit connested to it externally which reduces the pulse potential across the gap for a time. A last cathode has a large surface and the said pulse which failed to iire the penultimate cathode is arranged to arrive at this last cathode before it reaches the multipoint cathode owing to a delay circuit in the series between the pulse source and all cathodes but the last. The last cathode passes a surge of current due to its area, which drops the pulse voltage across a common lead and the delayed pulse has no time to nre the multipoint cathode, the gaps of which have not yet full deionised, and might otherwise have iired, but for the voltage drop due to the said current surge.

All gaps if the multipoint cathode will have been extinguished for two pulse periods before the next pulse and will have de-ionised to such a degree that only the shorter starting gap is in a position to re when that pulse arrives.

Another of the said devices described in my said application Serial No. 39,936, filed July 2l, i948, is similar to that last described but is arranged so that gaps iire once only during a pulse cycle. This device includes a tube similar to the above but including, in addition a long smooth cathode approximately parallel to the anode, and passing close to all the other discharge gaps. The end of this smooth cathode furthest from the starting gap has a projection towards the anode.

Pulses are applied between all cathodes and the anode and each pulse starts a gloviT from the projection on the smooth cathode to the anode; this glow spreads rapidly along the smooth surface of this cathode.

When it reaches the starting gap the ionisation due to the adjacent glow from the smooth cathode causes the starting gap tol re from the same pulse. 'I 'he other gaps were too large to re without the assistance of ionisation from an adjacent discharge from one of the sequence gaps in addition to the pulse potential and ionisation spread from the smooth cathode. v

All this happens during al single pulse and it is arranged that the glow can spread from end to end of the smooth cathode during about half the pulse duration.

At the end of the pulse the discharge in the starting gap and the glow, disappear.

A next pulse re-starts the glow which travels along the smooth cathode until it reaches the second gap next to the starting gap. This gap is vable to fire in the presence of the leading edge of the glow plus the residual ionisation from the recent starting gap discharge nearby. When it tires, the current inV a common cathode' load causes a voltage drop which prevents the starting gap from retiring when the glow reaches 1t due to the voltage dropped in this cathode load.

The process is continued along the gaps in the tube and the last pulse of the cycle causes a heavy discharge in the last gap having the large cathode surface, following the discharge in the penultimate gap controlled by the time constant circuit. The delay circuit used in the other device described in United States application Serial No. 39,936/48 is not required here since previous gaps once discharged will not relire due to the voltage drop in the common cathode load. When finally the pulse which fired the last electrode has ended the multipoint electrode will have been quiescent for two pulse periods, the penultimate gap will be prevented from ring by its time constant circuit and the last gap itself has a time constant circuit to prevent it firing again for several pulses. The succeeding pulse after that which lred the last gap will therefore find no gap able to nre, notwithstanding the advancing glow from the smooth cathode, except only the starting gap. The counting sequence thus re-starts.

It is, of course, essential that the direction of the spread of the ow along the smooth cath-l ode should be opposite the direction of advance of the discharges of the sequence gaps in the tube in order that one gap further on, shall lre off each succeeding pulse.

A third device described in my said co-pending United States application Serial No. 39,936/48 is similar in principle to that last described except that the tube is constructed so that the sequence gaps are provided by the intersection of two grid-like members with longitudinal sections of one at right angles to those of the other, the two being arranged in parallel planes spaced slightly apart. The advancing sequence discharges describes a to and fro course from one corner to the other of the two grid like mem bers, one of which forms the anode and the other the cathode of the tube. A grid like smooth cathode causes a flow following the course of the discharge gaps in reverse direction and a separate anode plate co-operates with the smooth cathode in providing the glow.

Two iinal electrodes are provided, and the device operates, in the Same manner as the last one described except for minor circuit differences due to the extra anode.

These devices can be arranged so that they will not commence to count when they have been quiescent for a time. long in relation to the pulse cycle time, except inthe presence of a pulse at level B.

In the rst device, this is arranged by dimensioning the starting gap so that it will not fire in the presence of a level A pulse alone or a level A pulse aided by the ionisation remaining after a missed pulse.

In the second and third devices, `where the assuma.

starting ngaps :remain extinguished for practically the whole pulse cycle time after its initial ring it is easier to arrange that the smooth cathode will not commence to glow, except in the presence of a level B pulse, when it has been quiescent for a time long in relation to the pulse cycle time but will commence to flow in the presence of a level A aided by the ionisation remaining at the commencement of a pulse succeeding a pulse during which it was glowing.

In using these devices `in linender units of the type described, they will full the role of selector Il primarily and once started by a calling pulse at level B will continue to count the pulses of all channels the starting gap requiring a pulse of level A, to cause it to fire and being unable to nre 'from a level C pulse, but all the other gaps being able to fire from a level A, B or C pulses when once the tube has started counting.

The functions of gate circuit It can readily be combined with these of l1, when these devices are used or alternatively it may be desirable to have a separate gate circuit which can only be opened from the quiescent state by a level B pulse, which remains open to all pulses so long as it is periodically triggered by a pulse of level A or greater on the calling channel but closes when the pulse of this channel falls to level C.

With such a separate gate circuit i5, it is easier to arrange Afor the clearance signal to stop the count of l1 than when the discrimination between A and C level pulses is confined to the starting gap, the others having to count pulses of all three levels.

As described in the said application Serial No. 39,936/48 these devices are arranged to deliver an output pulse on the firing of the last gap. For use in the role of l1 and or l in Fig. 1, however, the circuits and tubes need modification. The starting gap cathode point of all three types of tube must be connected to a separate leading out connection from that connected to the remaining points of the multipoint cathode and an output transformer winding must be connect ed between the starting gap cathode point and the common cathode lead instead of this transformer being connected in series with the lead to the last cathode.

Devices are known comprising a cold cathode gas tube and an accompanying circuit in which the tube contains two sets of discharge gaps' Amade between two multipoint cathodes and two anodes.

The arrangement of the electrodes in the tube is such that one set of gaps commences to lire in succession by the discharge across a vstarting gap, in the first instance, followed by the spreading of the discharge from gap to gap, from successive pulses, until on reaching the last gap, which has a cathode element with a large surface a current surge causes a voltage drop in a common lead which, in turn causes the discharges in this set of gaps to be extinguished. They are unable to discharge again for the time of 'a vnumber of pulses by a time constant circuit.

The starting gap of the second vset of discharge gaps is not smaller in size than the other gaps, as is that of the rst named set of gaps, and it is caused to re in the presence of the pulse -next succeeding that which fired the last gap kof the `first set of gaps, due to the ionisation of the gas therein, which it has received by migration from the last gap of the fust set -of gaps,

these 'two gaps being .near together vthough spaced somewhat further than the gaps of any one set of gaps. The second set of gaps then commences to fire in succession until, on reaching the last gap, the cathode element of which has a large area, a surge occurs which extinguishes rthe discharges. All this time the first set of gaps has been quiescent and is completely cle-ionised by the time the second set of gaps is extinguished. The last gap of the second set of gaps is placed near enough to the starting gap of the first set of gaps for the latter to become ionised on the discharge of the former and the sequence of discharges in the rst set of gaps therefore recommences on the arrival of the next pulse.

As described in the said application the output of the device is derived from the discharge in the last gap of the second set of gaps, but for use in the role of Il and/or I6 of Fig. 1 the output must be derived from the discharge in the rst or starting gap in the rst set of gaps.

The dimensions of the starting gap of the nrst set of gaps is such that only a pulse at level B can fire the gap when not ionized but that it can be red by a pulse at level A when ionised by the adjoining discharge of the last gap of the second set of gaps.

It must furthermore be arranged that, though the starting gap of the rst set of gaps will fire from a pulse of level A, once the tube has started to operate, it will not nre from a pulse of level C. The other `gaps however must all fire from pulses of level A, B or C so that the count is not lost when another channel sends a clearing pulse at level C1.

This device has the advantage over the counters described in my United States applications Serial Nos. 777,815 and '777,816 in that a pulse rate can be employed of which the interpulse interval is too short to enable the gas in the tube to be (ie-ionised since one set of gaps has virtually the whole time taken by the sequence of discharges in the second set vof gaps, to be come `cle-ionised. It is obvious that if gaps in a set of gaps were not de-ionised at the beginning of a cycle of pulses, the gaps might all lire at once, or the first gap to fire might be one other than the starting gap.

The'application of this 'device to a linender unit of the type described `is exactly similar to the application of the 'devices described in U. S. application, Serial Nos. 777,815 and 777,816 explained previously.

All the devices 'above referred to described in my said copending application No. '39,936/48, when used to fulfilv the 'functions of gate circuit I6 'and vselector ll of Fig." 1 can, by the use of a rectifier connected tothe vstarting gap, be made also to produce the neutralising pulse of 'pulse generator tu, since the B level pulse required to start the device from the quiescent state will cause a higher peak pulse voltage at the cathode than with subsequentAs-level pulses.

In the design of demodulatcr i8, to suit case 2 certain problems may arise which are not likely to vbe encountered in Ycase l.

Many demodulator devicesv rely en the provision of .a 'reference pulse with rrespect to which the variation of the signal pulse, due to time modulation, is measured. When such :a reference pulse is applied to the demodulators of the lineiinder units of a group, from a source common to the .l-inender units of the .group ther delay circuits through which the signal pulses pass may cause variations in the delay imposed upon the signal pulses so that, in extreme cases, they can drift from one side to another of the reference pulse causing great distortion of the demod'- ulated output. To guard against this each linefinder unit is individually balanced for delay, against the corresponding delay circuit inserted in the looped line between the incoming and outgoing connections of the linefinder unit to the line and this will in general stabilise the delay suiiiciently to obviate this occurrence. Where the delay circuits are lumped together on one or incre parts of the looped line however, more complicated methods of ensuring that the reference pulse is on the same side of the signal pulse, are necessary and this will usually involve the timing of the reference pulse being further away from the signal pulse. Any slow change or the mean position of the signal pulses in relation to the reference pulses will merely alter the direct current component of the dernodulated signal and will not cause distortion.

Where demodulators are used of the type described in my copending applications numbered 15,532/48 and 14,185/48, filed March 18, 1948, and March 11, 1948, respectively, the time between the reference pulse and the signal pulse is made to determine the width oi a widthmodulated pulse delivered to the tube. The tube must be designed to handle the greatest width of input pulse resulting from maximum modulation deviations of the signal pulse combined with maximum delay drift away from the reference pulse.

This may not always be convenient and it is possible to use a rectier gate demodulator of the type where the reference pulse opens a gate and permits a current to iiow and the signal pulse closes it and stops the current. The resulting currents may be ampliiied if their level is insuflicient and passed through a low pass filter (also forming part of demodulator i8), from which they will emerge at signal frequencies with the pulse component removed by the lter.

ln case 3 a demodulator of the type described in iny copending application Serial No. '756,262 may be used.

The design of unit I4 (Fig. 1) for cases 2 and 3 present special problems.

In case 2, the rectangular wave form derived from the pulse on any given channel, returning along portion 25 of the looped line, may wander too much due to modulation or the instability of the various delay circuits through which it passes, to be certain of always synchronising with some part of a later pulse of the same channel or of never over-lapping the pulse oi' an adjoining channel. This problem becomes more acute as the number of channels served by a lineiinder group, is increased. In case 3, as described above, the timing of the pulses is likely to vary, from modulation alone, between wide limits which may cause a returning pulse from line portion 25 to overlap the pulses of other channels and whilst the variation is limited by the fact that the time between a pulse on any channel and a later one of the same channel, with which the rst is required to synchronise on its return after making a circuit of the looped line, is large in relation to the time between peaks of adjacent cycles of the modulating signals, it is nevertheless possible for overlapping of channels to take place in I4 in the presence of sharp transients in the modulating signals.

The problem in both case 2 and case 3, may be solved by arranging for the transit time round the looped line to be appreciably less than the minimum period between successive pulses on any one channel and by storing information as to the nature o the pulse (whether level A, B or C) and releasing this information at the right moment for correct operation of device ill.

The blocking and extinguishing pulses can be provided from a source common to several line nder groups and consequently the use of comparatively elaborate hard tube circuits may be warranted.

Other means of achieving the required synchronisation, between returning pulses from line portion 25 and the pulses or" a later cycle from terminal are possible, for use in cases 2 and 3. For instance, one pulse of the pulse cycle may be reserved as a pilot pulse, the timing of this pulse depending on its return in a later pulse cycle at terminal l, of Fig. 1.

The pilot pulse could have any convenient distinguishing characteristic, for instance, it could be the iirst pulse of a pulse cycle, following a longer time interval between consecutive pulse cycles, than the normal inter-pulse inter- `val.

It could be arranged for the time difference between the returning pilot pulse and a pilot pulse from terminal l to be converted into a voltage proportional to the said time difference and for this voltage to control a variable delay circuit inserted somewhere in the looped line so as to bring the two pilot pulses into phase.

This may be achieved by incorpora-ting in some or one of the delay circuit, 4, 5, 6, 8, 9 or lil, or in an additional delay circuit, an inductive component or components having a magnetic circuit to which the said control voltage could be applied to saturate it. The inductance of the said components and consequently the delay period of the delai, circuit of circuits containing them, would thus vary in step with variation of the said control voltage.

Alternatively the said control voltage can be used to vary the restoring time of a chain of triggered discharge tubes inserted in the looped line. The maximum restoring time of each triggered rtube must be less than the minimum interpulse interval so that the nrst tube of the series would always be ready to receive a succeeding pulse.

Each successive tube in the train would inn pose a certain delay upon the pulses and the number of tubes required would depend on the overall delay required to bring returning pulses from line portion 25 into phase with later pulses at terminal l.

The circuit commonly known as the "E`ccles- Jordan circuit, could be used in this role.

The iso-called self-restoring Eccles-Jordan circuit would be most suitable. A pulse applied to the circuit, changes it from one stable condition to another stable condition and it reverts to the rst condition automatically after a period which can be adjusted by a con-trol voltage. 0n restoring an output pulse can be taken from the circuit, and seen after restoring the circuit is ready to be switched over by another pulse. Thus the output pulse is delayed in relation to the input pulse by an amount which can be varied by varying the said control voltage.

To produce delays large in relation to the used in a similar way in place of an Eccles-Jordan circuit.

It will thu-s be seen that this invention enables i' a comparatively large number of substations to be served by a lesser number of communication paths, in systems where such paths consist of diiTerent pulse time positions in a cycle of pulses.

What is claimed is:

1. A pulse transmission system comprising. means for selecting pulse times from a rst pulse -1 repetition time cycle, said means comprising a number of individual time pulse selecting means,

each adapted to respond to a marked pulse, an' associated local pulse circulation ring for use in. selection of marked pulse times and a trans-i mission channel for said first pulse cycle incoming to said circulation ring, said circulation ring including delay means such that a marked pulse completing a circuit of the ring without response meets the next incoming pulse of the same time position, and means for marking an incoming pulse if it coincidesl with a circulated marked pulse, means for creating pulses in a second independent pulse repetition time cycle, means for respectively transferring modulations carried by selected pulse times of said rst time cycle tc a corresponding number of pulse times in said second time cycle, said means comprising means for de-modulating the modulations on said selected pulse times, means for selecting pulse times in said second time cycle and means for modulating said last mentioned pulses with the product of said ie-modulating means.

2. A pulse transmission system as claimed in claim 1 further comprising means whereby the selecting means is responsive to a rst marking modulation to cause selection of a pulse time and to a second marking modulation of the same pulse time to cause disconnection of said pulse time.

3. A pulse time transmission system, as claimed in claim 29, in which the system comprises a telecommunication exchange comprising a plurality of sub-stations and a main station, means at said sub-stations for modulating pulses of the rst pulse repetition time cycle with calling or selecting and clearing signals, and in which the means for selecting pulse times is located at said main station and is adapted for responding to a calling or selecting modulation to cause selection of a pulse time, and for responding to a clearing modulation of the same pulse time to cause disconnection of said pulse time.

4. A pulse transmission system as claimed in claim 1 in which said selecting means comprises a number of individual time pulse selecting means each adapted to respond to a marked selection lpulse, and in which means is provided Whereby a marked pulse is continuously offered to all of said individual selecting means until accepted by one 0f said individual selecting means.

5. A telecommunication transmission system as claimed in claim 4 in which each individual selecting means is individually associated with a predetermined position in the second time cycle.

ALEC HARLEY REEVES.

REFERENCES CITED The following references are of record in the le o1' this patent:

UNITED STATES PATENTS Number Name Date 2,418,116 Grieg Apr. 1, 1947 2,420,374 Houghton May 13, 1947 2,421,017 Deloraine May 27, 1947 2,447,233 Chatterjea Aug. 17, 1948 2,506,613 Ronsom May 9, 1950 2,509,218 Deloraine May 30, 1950 2,524,776 Deloraine Oct. 10, 1950 2,524,861 Wallace et al. Oct. 10, 1950 FOREIGN PATENTS Number Country Date 600,258 Great Britain Apr. 5, 1948 

